The present invention relates to an adhesive formulation which can be rapidly cured and which is suitable for bonding semiconductor devices to a substrate. More particularly, the invention relates to an adhesive formulation for bonding semiconductor chips, also known "dies", to a lead frame and, still more particularly, to an adhesive formulation which may be dispensed in small amounts at high speed and with sufficient volume control to enable the adhesive to be deposited on a substrate in a continuous process for the production of bonded semiconductor assemblies.
Organic adhesives have been used to bond IC chips to metal lead frames in plastic packages. Die attach adhesives which have been employed for this purpose include either epoxy or polyimide material filled with precious metal. However, epoxy and polyimide suffer from various shortcomings.
Epoxy adhesives generally have low glass transition temperature, i.e., less than 150.degree. C. for liquid epoxy, and have a high degree of ionic contaminants. Although epoxy can be rapidly cured, the low glass transition temperature and ionic contaminants adversely effect the reliability of the material. Polyimide adhesives generally contain solvents and require a lengthy curing time, often exceeding two hours.
As dies become increasingly larger, the need for a suitable, rapidly-curing adhesive has increased. To accommodate the growing need for bonding dies to substrates in a continuous manner at high production speeds, it is desirable that the adhesives have a glass transition temperature that exceeds 200.degree. C., contains little or no solvents or diluents, has a very low level of ionic contaminants, provides good adhesion at room temperature and at temperatures greater than 150.degree. C. and, importantly, is able to be cured rapidly, i.e., in less than 5 minutes at 200.degree. C., preferably less than 2 minutes at 200.degree. C. The ability of an adhesive to be rapidly cured is especially important since the curing rate affects the usefulness of the adhesive in continuous processes for production of bonded semiconductor assemblies.
In accordance with the present invention a rapidly-curing adhesive formulation suitable for bonding semiconductor devices to a substrate may be formulated using a cyanate ester such as "AROCY L10" available from Hi-Tek Polymers, Inc. of Louisville, Ky. This cyanate ester is a liquid dicyanate monomer such as described in Shimp U.S. Pat. No. 4,785,075, Craig, Jr. U.S. Pat. No. 4,839,442 and in an article entitled New Liquid Dicyanate Monomer for Rapid Impregnation of Reinforced Fibers, by Shimp and Craig, Jr., presented at the 34th International Sampe Symposium in Reno, May 8-11, 1989, the disclosures of which are all, individually and collectively, specifically incorporated herein by reference.
The cyanate esters are resins of a family of aryl dicyanate monomers and their pre-polymer which contain a reactive cyanate functional group. When heated the cyanate functionality undergoes exothermic cyclotrimerization reaction to form triazine ring connection units which result in gelation and formulation of thermo set polycyanurate plastics.
It is known that metal catalysts such as napthenates, acetylacetonates or chelates of zinc, copper, and cobalt dissolved in alkylphenols, such as nonylphenol, will affect the curing time of the cyanate ester in connection with fiber impregnation, as described in the aforementioned Shimp and Craig, Jr. article. However, the practical use of cyanate esters in formulating adhesive films for bonding semiconductor devices to substrates, especially in continuous processes, has not heretofore been suggested or described. Furthermore, it is not at all obvious that cyanate ester resins should be used in the formulation of die attach adhesives requiring rapid curing. When heated, cyanate esters undergo exothermic cyclotrimerization reactions liberating heat by this reaction of 24 kcal per gram-equivalent, which is enough heat to raise the batch temperature as much as 390.degree. C. under adiabatic conditions. Adiabatic reactions are created when small quantities, such as 200 grams, are over-heated, or over-catalyzed so as to release all of the reaction energy in a short time, i.e., a few minutes or less. The excess temperature may cause a dangerous runaway reaction and thermal decomposition of the material. As a result, the cure schedules that are recommended are usually very lengthy, up to several hours with carefully controlled curing temperatures. These conditions are described in the aforementioned Shimp and Craig, Jr. article and would suggest that cyanate esters are unsuitable for the practical and general use to which the present invention is applicable.
The following are typical cure schedules recommended by Hi-Tek Polymers, INc. for the cyanate ester resin:
Minutes to gel at 104.degree. C. --15 minutes PA1 Hours at 177.degree. C. to cure--one hour PA1 Hours at 210.degree. C. to cure --one hour PA1 Hours at 250.degree. C. to cure --two hours
As can be seen, therefore, the curing schedule heretofore recommended for cyanate ester is very lengthy and unsuited for semiconductor bonding processes, especially high speed continuous production processes.
Notwithstanding the foregoing, it has been discovered that the use of cyanate ester resin in a rapidly curing die attach adhesive formulation is feasible under the following conditions: (1) including a filler with high thermal conductivity in the adhesive formulation; (2) applying the adhesive formulation as a thin bonding layer, e.g., 5 mil or less, preferably 1 or 2 mil or less; (3) dispensing small amounts of adhesive, e.g., less than 2 mg at a time, during continuous processes for production of bonded semiconductor assemblies; and (4) applying the adhesive for bonding to surfaces made of material having a high thermal conductivity, e.g., ceramics, lead frames, copper, alloy 42, etc.
When the foregoing conditions are present, the cyanate ester containing adhesive can be cured rapidly without detrimental effects since all the heat of reaction can be removed by the high thermal conductivities of the adhesive loaded with thermally and/or electrically conductive filler and the surfaces to which the dies are to be bonded which may also have a high thermal conductivity, and especially with the use of relatively thin adhesive films that result in a thin bond line thickness.